This invention relates to a fluid driven, two-post lift, more specifically, to an improved hydraulic circuit for operating a fluid driven two-post lift.
Two-post lifts employing cylinders operated by hydraulic fluid under pressure are well known in the prior art. For example, U.S. Pat. No. 4,500,071 to Bagwell et al. provides a vehicle lifting apparatus employing a load bearing means operatively connected to a pair of lifting units. The lifting units employ a master cylinder and slave cylinder arranged to effect simultaneous movement of the cylinders.
U.S. Pat. No. 4,505,455 to Beatty also provides a hydraulically operated double-post lift assembly. The U.S. Pat. No. 4,892,028 to Stivers discloses a fluid operated circuit for controlling a dual-post hydraulic lift assembly.
There are at least three major problems associated with prior art two-post hydraulic lift systems. First, if there is an obstruction under one side of the lift, during downward movement of the lift the unobstructed side can continue in its downward movement causing the load bearing member to tilt to one side. This could result in a catastrophic dumping of the load.
Second, coordination of the master and slave cylinders is based on an optimum fluid displacement ratio and hydraulic pressure. Although theoretically sound, the units do not always operate in this manner. The system, due to wear, may develop fluid leaks or air may accumulate in hydraulic lines. Furthermore, wear on the cylinder may change the fluid displacement ratios between the master and slave cylinders In any event, the master and slave cylinders may not operate in a coordinated manner. The master cylinder may rise or descend more quickly than the slave cylinder causing the load bearing member to rise or descend unevenly Often, this phenomenon is more pronounced if the unit sits idle for awhile, for example, overnight. Air in the hydraulic lines, which is ordinarily dispersed as small bubbles throughout the fluid, tends to accumulate in one large bubble when the unit sets idle. Although hydraulic fluid is minimally compressible, on the order of 0.5% per 1000 psi, a large air bubble is quite compressible and can disrupt coordinated operation of the master and slave cylinders.
A third major problem associated with the prior art two-post lifts is lift failure, or rapid descent of the lift due to a drop in fluid pressure. This phenomenon is caused when, for example, the hydraulic lines spring a leak with a load in the elevated position.
There have been a number of attempts made to remedy the major problems associated with prior art two-post lifts For example, in the above mentioned U.S. Pat. No. 4,505,455 to Beatty, the inventors provided a safety latch means to lock the moveable elements in place to block accidental descent of the load bearing unit upon sudden loss of fluid from the cylinders. This safety latch feature remedies only one of the triumvirate of major problems associates with prior art lifts. Moreover, it does not solve the problem of rapid descent if the latches are disengaged and the cylinders are in some stage of downward or upward movement.
U.S. Pat. No. 4,892,028 to Stivers attempts to remedy the problems associated with both an obstruction under one side of the unit as well as the major problem associated with the lack of coordination between the cylinders. Stivers employs a system of pulleys and chains with an integrated hydraulic system. The hydraulic system employs a plurality of pilot valves designed to halt the downward movement of one side of the lift device if the other side is obstructed. This system is complex, having a plurality of parts including the chain and pulley system, as well as the associated hydraulic circuitry. Moreover, if the hydraulic circuit fails, the chains may slacken on one side or the other causing a tilting of the load bearing member and dumping of the load.
Stivers employs a means for re-leveling the lift, side-to-side, should the load bearing member become out of level due to leaking fittings or seals. Stivers employs slots at the fully lowered and fully raised positions of the master cylinder in order to allow fluid to flow around the piston to the slave cylinder to equalize hydraulic pressure and level the lift when the lift is either fully raised or lowered. This automatic leveling feature introduces an inherent out-of-level condition by allowing fluid to flow around the master cylinder piston for a short period of time as the master cylinder piston is moving past the slot location. The lower slot can, in fact, add to the out-of-level condition of the lift during raising operation.